Fulcrum was engineered to work in a completely disconnected state, meaning without any connection to cellular or wifi networks. This is a beneficial feature for working in areas where data roaming charges may apply or even when collecting data in remote environments such as rural areas. In a previous post, MOBAC was mentioned as a external tool for generating offline maps from sources like MapQuest Open, OpenStreetMap, and CloudMade. These are excellent data sources used for reference maps, but suppose you need offline imagery for reference due to a lack of mapping data in a remote area. TileMill has the ability to generate .mbtile files with imagery which can then be used for offline reference maps on Fulcrum mobile devices.
In the U.S. many imagery repositories are maintained by each state. For instance, the state of Florida maintains a public online geospatial data portal called, LABINS (Land Boundary Information Systems) which is maintained by the Bureau of Survey and Mapping and the Florida Resources and Environmental Analysis Center (FREAC). High resolution digital imagery can be extracted and downloaded from the Florida Images Inventory.
- USGS National Map allows extraction of .tiff imagery.
- Other imagery data sources can be found here.
QGIS & GDAL
GDAL is a translator library for raster geospatial data formats. Once your GDAL binary is installed you can the fetch the GDAL plugin from QGIS. Once you have your raster data (imagery) you’ll need to convert it to .tiff for use in TileMill. This can done be using gdal_translate to convert formats. For the best rendering performance in TileMill, reproject your .tiff image using gdalwarp to SRS:900913 (Spherical Mercator).
Create a new project in TileMill and add your .tiff image as a new layer. You can even overlay shapefiles that you may have of the data collection area to compliment your offline reference imagery. Here I’ve added a parcel shapefile that shows the property extent of where the data collection is to occur with some simple CartoCSS styling. When you’re ready to export your offline map, select the appropriate zoom levels and map bounds.
When you’re ready to add your newly created offline map to your Fulcrum device you can upload it right to your Fulcrum account in the map uploader or use the iTunes File Sharing feature to add it directly to your device. Offline imagery maps in Fulcrum provide data collectors with a visual reference tool to accurately and efficiently gather data in the field.